1. Field of the Invention
This invention relates to toners used for the dry development of an electrostatic latent image in an electrophotograph.
2. Description of the Prior Art
There have heretofore been known several dry developing methods including a method using a two-component developer composed of toner particles mixed with carrier particles such as glass beads or magnetic powder, and a method using a one-component toner composed of toner particles imparted with magnetism. Recently, there has been proposed a method using a one-component nonmagnetic toner which is excellent in environment resistance.
These toners have been, in most cases, prepared by mixing, heating and melting thermoplastic resins, colorants such as pigments or dyes and additives such as wax, plasticizers, charge-controlling agents and the like; kneading the pigments in the form of secondary agglomeration under the application of intense shearing force thereto; uniformly dispersing, if necessary, magnetic powder to the mixture to obtain a uniform composition; cooling and comminuting the composition; and then classifying the resulting particles to obtain desired toner particles.
However, the toner particles so obtained are qualitatively disadvantageous in that they are not uniform in size and shape and are generally amorphous, so that the individual particles have different frictional charging characteristics, thus causing their staining or scattering within a machine concerned. In addition, the toner particles have so low flowability that it becomes difficult to supply them smoothly with many troubles being undesirably involved. On the other hand, from the standpoint of a process for the production thereof, there are several problems that much energy is required for the kneading step and that the classification undesirably needs a number of processing steps.
To avoid this, there have been proposed attempts to obtain spherical toners by a spray drying or suspension polymerization process. However, the former process requires proper selection of resins which are soluble in a solution and presents a problem as to an offset phenomenon on fixing drum. The latter process raises problems as to blocking and offset phenomena and is therefore not industrially used.
Conventional toners have the common disadvantage that a colorant and a charge controlling agent, which exhibit their characteristic properties on the toner surface and are relatively expensive as starting materials for the toners, are uneconomically contained not only in the surface portion of the toner but also in the inside thereof.